Method and apparatus for detecting a gas pocket using ultrasound

ABSTRACT

Existing gas pocket detection approaches are based on visual observations of B-mode ultrasound images showing comparisons between normal soft tissue and gas pockets, which are time-consuming and dependent on operator experience. The present invention proposes an ultrasound system and a method of detecting a gas pocket. The ultrasound system comprises: an ultrasound probe ( 110 ) for transmitting an ultrasound signal toward the ROI and acquiring an ultrasound echo signal reflected from the ROI along a plurality of scanning lines; an obtaining unit ( 130 ) for obtaining a second harmonic component of the ultrasound echo signal for each depth of a plurality of depths along each scanning line of the plurality of scanning lines; and a deriving unit ( 140 ) for deriving a change in a center frequency of the second harmonic component along with the depth.

FIELD OF THE INVENTION

The present invention relates to ultrasound imaging, particularly to amethod and an apparatus for detecting a gas pocket using ultrasound.

BACKGROUND OF THE INVENTION

Trauma is the leading cause of death in the United States for men andwomen under the age of 45 years and the fourth overall cause of deathfor all ages. Trauma also has a substantial economic impact on thehealth care system, accounting for over one-third of all emergencydepartment visits and resulting in over $80 billion per year in directmedical care cost, for example, in 2007, over 180 000 people died oftrauma, and abdominal injuries contributed to a large number of thesedeaths.

Pneumoperitoneum is a condition in which a free gas pocket or tinyamount of free gas or air is trapped within the abdominal cavity but notcontained in a hollow viscus. Identifying abnormal intra-abdominal gaspockets or collections may be critically important in establishing anaccurate diagnosis. Increasing evidence supports that ultrasound imagingis a very useful tool for diagnosis of pneumoperitoneum with abnormalair/gas patterns because of its high accuracy and superiority ascompared to plain X-ray radiography. The sonographic air can be outlinedas comprising two categories: physiological air or normal air; andpathologic air or abnormal air. Physiological air is air in thegastrointestinal tract and lungs (air projecting into the abdominalcavity).

Bedside ultrasound or Point-of-care Ultrasound is widely used inemergency medicine for initial screening and enables selection ofhemodynamically unstable traumatic patients with severe hemoperitoneumfor immediate surgery. The detection of intraperitoneal free air or anintraperitoneal gas pocket is very helpful for bedside diagnosis ofacute abdomen and trauma patients. The detection of a gas pocket maysupport doctors to assess, although not in a direct way, if there is anabnormal gas pocket from blunt abdominal trauma or from acute abdomen in(a) pre-hospital settings, (b) initial evaluation in the emergency room,and (c) follow-up after some treatments.

The sonographic appearance of gas pockets is due to total ultrasoundreflection (a strong reflector) at the interface of soft tissue and gaspocket (air). This reflection is accompanied by reverberation ofultrasound between the gas pocket and the ultrasound probe. Therefore,sonographic images for gas pockets typically appear as high-amplitudeechoes (brightness area in the image) with distal artifactualreverberation echoes referred to as “dirty shadowing”; smallreverberation artifacts have a characteristic comet-tail appearance.Small gas pockets may show little or no distal reverberation artifactswith standard abdominal transducers. The optimal probe position fordetecting intraperitoneal free air after blunt abdominal trauma is inthe right paramedian epigastric area in the longitudinal direction.

Muradali et al. (“A specific sign of pneumoperitoneum on sonography:enhancement of the peritoneal stripe”, AJR, 1999, Vol. 173:1257-1262)studied the signs of pneumoperitoneum from animal models, which werethen confirmed in patients who had undergone laparoscopy. This kind ofcharacteristics of gas pockets in the ultrasound images is calledEnhancement of the Peritoneal Stripe Sign (EPSS). This EPSS is furtherconfirmed by recent prospective study of six hundred consecutivepatients with acute abdominal pain. The EPSS had a sensitivity of 100%,a specificity of 99%, a positive predictive value of 87.5% and anegative predictive value of 100%. Therefore, EPSS is recommended as areliable and accurate sonographic sign for the diagnosis ofpneumoperitoneum through visual observation.

Conventional gas pocket detection based on ultrasound imaging works wellif gas pockets are quite large, i.e. large enough to produce enhancementof the peritoneal stripe sign (EPSS). It takes a long time fornon-experienced users to identify this kind of sign (EPSS) fromultrasound images. It is really difficult for emergency physicians toidentify all gas pockets in an examination time of around 5 minutes forthe whole abdomen even if they know the optimal probe position fordetecting intraperitoneal free air after blunt abdominal trauma in theright paramedian epigastric area in the longitudinal direction.

In summary, larger gas pockets may appear as bright, highly echogenicstripes or lines with distal reverberation and dirty shadowing artifactsor comet-tail artifacts which may even obscure the underlying abdominalorgans. Smaller gas pockets can appear as bright punctuate foci withoutring-down artifacts and shadowing within the intestinal lumen, but maynot have reverberation within the image. Ultrasound imaging is superiorto chest X-rays for diagnosing intraperitoneal free air; quantities assmall as 1 ml to 2 ml of intraperitoneal free air can be detected byultrasound. However, the detection of intraperitoneal free air might bedifficult even for an experienced sonographer in emergency situationsunder difficult patient conditions.

Thus, most of the existing gas pocket detection approaches are based onvisual observations of B-mode ultrasound images showing comparisonsbetween normal soft tissue and gas pockets. Such existing approaches aretime-consuming and the accuracy is very dependent on the operator'sexperience.

SUMMARY OF THE INVENTION

Therefore, it would be advantageous to provide an improved method andapparatus or system for detecting a gas pocket in a Region-of-Interest(ROI).

According to an embodiment of a first aspect of the present invention,there is proposed an ultrasound system for detecting a gas pocket. Theultrasound system comprises: an ultrasound probe for transmitting anultrasound pulse toward the ROI and acquiring an ultrasound echo signalreflected from the ROI along a plurality of scanning lines; an obtainingunit for obtaining a second harmonic component of the ultrasound echosignal for each depth of a plurality of depths along each scanning lineof the plurality of scanning lines; and a deriving unit for deriving achange in a center frequency of the second harmonic component along withthe depth. The term “depth” refers to the penetration depth of theultrasound signal. The term “scanning line” is also called “receivingline” in the field of ultrasound imaging.

The inventors of the present application recognize that the centerfrequency of the second harmonic component of the ultrasound echo signalis expected to decrease along the penetration depth at a certain rate inthe area where the normal soft tissue is present, whilst it is expectedto change in a different manner along the penetration depth in the areawhere the gas pocket is present. For instance, along the scanning line,the decrease of the center frequency of the second harmonic component(also referred to as the attenuation of the center frequency) due to thepresence of the gas pocket at a specific depth is assumed to be greaterthan the decrease due to the presence of the soft issue at the samedepth. Therefore, the inventors of the present application propose theaforementioned method. A person skilled in the art will appreciate thatthe term “area” is not intended to be restricted to a two-dimensionalspatial region within the field of view of the ultrasound probe, andparticularly, in case that the ultrasound probe is a 3D ultrasound probefor imaging a 3D ultrasound image, the term “area” can be understood asa two-dimensional or three-dimensional spatial region within the fieldof view of the ultrasound probe.

By means of the proposed method, the second harmonic component of theultrasound echo signal reflected from the ROI is obtained and the changeof the center frequency of the second harmonic component of the echosignal along with the depth is derived. Then, the gas pocket can bemanually or automatically detected based on the derived change in thecenter frequency of the second harmonic component along with the depth.In particular, the change in the center frequency of the second harmoniccomponent along with the depth can indicate the trend of the centerfrequency along with a plurality of depths (namely the trend along withthe propagation or penetration direction of the ultrasound signal). Incomparison with the decrease of the center frequency at a depth, thechange in the center frequency of the second harmonic component reflectsthe trend of the decrease of the center frequency along with the depthand is expected to be more reliably used for detecting a gas pocket,especially a macro gas pocket.

According to an embodiment of a first aspect of the present invention,the second harmonic component is obtained by means of a pulse inversiontechnique.

It is appreciated by the person skilled the art that in the case ofpulse inversion technique, two transmissions (i.e. one positive pulsetransmission and one negative transmission) and two correspondingreceptions are conducted for each scanning line. By using pulseinversion technique, the boundary of a possible gas pocket and darkareas behind the gas pocket may be clearly visible, and also thecontrast between the boundary of different tissues is improved.

According to an embodiment of a first aspect of the present invention,the second harmonic component is obtained by means of band-passfiltering. It is appreciated by the person skilled in the art that inthe case of using band-pass filtering (hereinafter referred to asband-pass filtering approach), the second harmonic component can bederived from one transmission and one corresponding reception along eachscanning line. Thus, in comparison to the pulse inversion technique, theadvantages of the band-pass filtering approach include an increasedframe rate and/or less sensitivity to motion artifacts.

According to an embodiment of a first aspect of the present invention,the deriving unit is configured to derive, for each scanning line, afrequency-depth curve representing the relationship between the centerfrequency and the depth, and to derive the slope of the frequency-depthcurve at each depth of the plurality of depths.

In other words, the frequency-depth curve represents the centerfrequency with respect to the depth, and hence its slope represents therate of the change of the center frequency along with the depth.

According to an embodiment of a first aspect of the present invention,the frequency-depth curve is smoothened by averaging over a secondpredetermined number of ultrasound scanning lines. For example, thecenter frequency at a depth along a scanning line in the smoothenedfrequency-depth curve is computed as the moving average window (alsoknown as the sliding average) with a size equal to the secondpredetermined number of scanning lines. Given a sequence of samples, themoving average of a sample is known to be defined as the average valueof all samples within a window containing that sample.

By means of said smoothening over a number of ultrasound scanning lines,undesirable distortions caused by noises and/or disturbances can bereduced thanks to the averaging effect.

According to an embodiment, the ultrasound system further comprises adisplay unit for generating an ultrasound image representing the derivedchange in the center frequency along with the depth, and for displayingthe ultrasound image.

In this way, users, such as doctors or sonographers, can obtainknowledge about the change of the center frequency along with the depthby viewing the ultrasound image, and judge whether there is a gas pocketbased on the obtained knowledge. For example, if it is observed that thecenter frequency drops sharply along with the depths at a location, theusers can infer that there is a high possibility that a gas pocketexists at that location.

According to an embodiment, the ultrasound system further comprises adetecting unit for detecting a gas pocket based on the change of thecenter frequency along with the depth, and a display unit fordisplaying, in an ultrasound image, an indicator for indicating thedetected gas pocket.

In this way, the results of gas pocket detection are directly presentedto the users. The indicator for the detected gas pocket can be displayedin various types of ultrasound images, such as a B-mode ultrasoundimage, an ultrasound image illustrating the change of the centerfrequency along with the depth, or a combination thereof.

According to an embodiment, the second harmonic component is obtained bymeans of pulse inversion technique, and the gas pocket is detected at adepth if an amount of the change in the center frequency along with thedepth exceeds a first predetermined threshold at the depth. In otherwords, the gas pocket can be detected in the case of a sharply changedcenter frequency of the second harmonic component along with the depth,whereas soft tissue can be detected in the case of a nearly linearlychanged center frequency of the second harmonic component along with thedepth.

According to another embodiment, the second harmonic component isobtained by means of a band-pass filtering approach, and the detectingunit is configured to obtain a first determining result indicatingwhether the change in the center frequency along with the depth in anarea forms a bell shape, and to determine whether a gas pocket ispresent in the area based on the first determining result. In otherwords, if a gas pocket is present in an area, the curve for the centerfrequency of the second harmonic component with respect to the depth isexpected to increase initially at the gas-pocket boundary and thendecrease sharply, which combination of generation and transmissionprocess forms a bell-shape curve. The person skilled in the art willunderstand that the change of a parameter forms a bell shape if theparameter first increases and then decreases.

According to the understanding of the inventors of the presentinvention, the high frequency components of the produced echo signalincluding the second or higher order harmonic component will becomestronger when the ultrasound signal hits the boundary of a gas pocket,and then decrease sharply along the depth due to the strongerattenuation caused by the same gas pocket.

According to another embodiment, the detecting unit is configured todetermine whether a gas pocket is present between a first depth and asecond depth along a scanning line based on a second determining result,wherein the second depth is deeper than the first depth, and the seconddetermining result indicates, along the scanning line, whether thechange in the center frequency along with the depth is greater than anon-negative second predetermined threshold at the first depth and lessthan a non-positive third predetermined threshold at the second depth.In an example, both the second predetermined threshold and the thirdpredetermined threshold are zero. In this case, a gas pocket is detectedif a positive slope is followed by a negative slope in a frequency-depthcurve representing the relationship between center frequency of thesecond harmonic component and depth.

According to an embodiment, the detecting unit is configured to obtain athird determining result indicating whether the intensity of theultrasound echo signal at the first depth is lower than a fourththreshold, and to determine whether a gas pocket is present between thefirst depth and the second depth based on the second determining resultand the third determining result. In an example, the fourth threshold isdetermined based on the average intensity of the ultrasound echo signalin the whole ROI. If the ultrasound echo signal in an area is far belowthe average intensity in the whole ROI, the derived change of the centerfrequency of the second harmonic component along with the depth may beso contaminated by noise and/or artifacts that the detecting basedthereon is not reliable anymore. Thus, the detection accuracy can beimproved if detecting the gas pocket is further based on the intensityof the ultrasound echo signal. In various embodiments, the intensity ofthe ultrasound echo signal can be represented by the intensity of thefundamental component, or the second harmonic component, or thefrequency component of the ultrasound echo signal.

According to an embodiment of a second aspect of the present invention,there is proposed a method of detecting a gas pocket. The methodcomprises: transmitting an ultrasound signal toward the ROI andacquiring an ultrasound echo signal reflected from the ROI along aplurality of scanning lines; obtaining a second harmonic component ofthe ultrasound echo signal for each depth of a plurality of depths alongeach scanning line of the plurality of scanning lines; and deriving achange in a center frequency of the second harmonic component along withthe depth.

Other objects and advantages of the present invention will become moreapparent and can be easily understood with reference to the descriptionmade in combination with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

The present invention will be described and explained hereinafter inmore detail in combination with embodiments and with reference to thedrawings, wherein:

FIG. 1 is a schematic diagram of an ultrasound system for detecting agas pocket in accordance with an embodiment of the present invention;

FIG. 2 shows the respective normalized spectra for a regular ultrasoundecho signal and its pulse inversion version;

FIG. 3a and FIG. 3b show typical frequency-depth curves for a gas pocketand normal soft tissue (liver);

FIG. 4a shows a grey B-mode ultrasound image of a region of interest(ROI), FIG. 4b shows a colorized parametric ultrasound image of the ROI,and FIG. 4c shows the colorized parametric ultrasound image overlaidwith the grey B-mode ultrasound image;

FIG. 5a shows a grey B-mode ultrasound image of a region of interest(ROI), FIG. 5b shows one colorized parametric ultrasound image of theROI representing the center frequency of the second harmonic component,FIG. 5c shows another colorized parametric ultrasound image of the ROIrepresenting the slope of the center frequency of the second harmoniccomponent, FIG. 5d shows the frequency-depth curve along the 150^(th)line in the ultrasound image of FIG. 5b , and FIG. 5e shows thefrequency-depth curve at a depth of 4 cm in the ultrasound image of FIG.5b ; and

FIG. 6 shows a flowchart for an exemplary method of detecting a gaspocket.

The same reference signs in the figures indicate similar orcorresponding features and/or functionalities.

DETAILED DESCRIPTION

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn to scale forillustrative purposes.

FIG. 1 shows a schematic diagram of an ultrasound system 100 fordetecting a gas pocket in a Region-of-Interest (ROI) in accordance withan embodiment of the present invention. As shown in FIG. 1, theultrasound system 100 includes an ultrasound probe 110 for transmittingan ultrasound signal toward the ROI and acquiring an ultrasound echosignal reflected from the ROI. The ultrasound echo signal may bepre-processed to reduce noise and/or motion artifacts.

The pre-processed signals are coupled to an obtaining unit 130. Theobtaining unit 130 is configured to obtain a second harmonic componentof the ultrasound echo signal for each depth of a plurality of depths ona scanning line in the ROI.

According to an embodiment of a first aspect of the present invention,the second harmonic component is obtained by means of a pulse inversiontechnique. As known by the person skilled in the art, positive pulsetransmission and negative pulse transmission are successively performedalong with each scanning line, and two corresponding RF (radiofrequency) lines are received, respectively. The pulse inversion versionof the ultrasound echo signal is the sum of the two RF lines, namely thesum of the received echo of the positive pulse transmission and thereceived echo of the negative pulse transmission. Generally, when theultrasound signal encounters a gas pocket, it alternately compresses thegas pocket in the positive pressure phase and expands it in the negativepressure phase. However, the extent to which the gas pockets arecompressed during the positive pressure phase does not correspond to theextent of expansion in the negative pressure phase. In other words, thecompression and expansion are not symmetrical and thus harmoniccomponents are produced.

In an exemplary embodiment, the received ultrasound echo signalcomprises a raw 512-line RF signal collected along 256 scanning lines intissue harmonic mode, 256 lines of which are the echoes of the positivepulse transmission and the other 256 lines of which are the echoes ofthe negative pulse transmission. Then, a new 256-line RF signalindicating the pulse inversion version is derived from the raw 512-lineRF signal. The new 256-line RF signal is then pre-processed to reducenoise or motion artifacts.

FIG. 2 shows the respective normalized spectra for a regular ultrasoundecho signal and its pulse inversion version. It can be seen that theenergy of the second harmonic component obtained from the pulseinversion at a bandwidth of [2.5 to 5] MHz is around 70% to 80% of thetotal energy from the pulse inversion signal. Therefore, the secondharmonic component can be generally selected as the bandwidth forspectral analysis.

Harmonic imaging relies on transmitting at a fundamental frequency andforming an image from harmonic components of the ultrasound echo signal,where filters are used to remove the fundamental component. Pulseinversion technique can separate the fundamental component of the gaspocket echoes from the harmonic components even when they overlap. Inpulse inversion technique, any linear target (e.g., tissue) thatresponds equally to positive and negative pressures will reflect equallyand its respective echo signals will cancel out.

Gas pockets respond differently to positive and negative pressures anddo not reflect identical inverted waveforms. When these echo signals areadded, they do not cancel out completely. By using the pulse inversiontechnique, the boundary of possible gas pockets and dark areas behindthe gas pockets becomes clearly visible and also the boundary ofdifferent tissue exhibits better contrast. By comparison, conventionalB-scan images do not show dark areas behind a gas pocket very clearly.As such, a B-scan image from pulse inversion (major second harmoniccomponent) shows better contrast than that from conventional ultrasound.As such, for further non-linear analysis, the second harmonic componentmentioned above may be obtained by means of pulse inversion technique.

Examples of signals used for further non-linear analysis are not limitedto second harmonic components obtained by means of pulse inversiontechnique. For example, regular second harmonic components can also beselected for non-linear analysis.

According to another embodiment, the second harmonic component isobtained by means of a band-pass filtering approach. In particular, theobtaining unit 130 comprises a band-pass filter. Alternatively, theobtaining unit 130 can comprise a low-pass filter and a high-passfilter.

As known by the skilled person, fundamental component, second harmonicsand even higher order harmonics (if the bandwidth of the ultrasoundprobe is wide enough to have higher order harmonics) are all present inthe ultrasound echo signal, and there is spectrum overlapping betweenfundamental component and 2nd harmonics as well as between secondharmonics and higher order harmonics.

In an embodiment, the ultrasound echo signal along each scanning line iscollected and then a band-pass digital filter with a lower cut-offfrequency f1 and an upper cut-off frequency f2 is applied to obtain thesecond harmonic component of the ultrasound echo signal. Alternative toa band-pass digital filter, a high-pass digital filter with the lowercut-off frequency f1 and a low-pass digital filter with the uppercut-off frequency f2 can be applied. The lower cut-off frequency and theupper cut-off frequency can be predetermined based on the centralfrequency of the ultrasound signal. In an example, the central frequencyof the ultrasound signal transmitted by the ultrasound probe is 2 MHz,and hence the central frequency of the second harmonic component isaround 4 MHz. Thus, the lower cut-off frequency f1 can be set in therange from 2.7 MHz to 3.2 MHz, preferably at 3.0 MHz, and the uppercut-off frequency f2 can be set in the range from 5.0 MHz to 5.5 MHz,preferably at 5.0 MHz.

In an exemplary embodiment, a raw 512-line RF signal is collected along256 scanning lines in tissue harmonic mode, 256 lines of which are theechoes of the positive pulse transmission and the other 256 lines ofwhich are the echoes of the negative pulse transmission. Either the 256lines of the positive pulse transmission or the 256 lines of thenegative pulse transmission are selected, referred to as a new 256-lineRF signal. The new 256-line RF signal is then pre-processed to reducenoise and/or motion artifacts. Thereafter, the obtaining unit 150obtains the second harmonic component from the pre-processed new256-line RF signal.

Referring back to FIG. 1, the obtained second harmonic component, forexample the pulse inversion version of the ultrasound echo signal, iscoupled to a deriving unit 140. The deriving unit 140 is configured toderive a change in a center frequency of the second harmonic componentalong with the depth.

The detection unit 130 and the deriving unit 140 can be implemented as asingle processor or separate processors.

An ultrasound system normally comprises one or more processors such as asignal processor coupled to a beamformer and a B mode processor coupledto the signal processor. The signal processor can process the receivedecho signals in various ways, such as bandpass filtering, decimation, Iand Q component separation, and harmonic signal separation which isapplied to separate linear and nonlinear signals so as to enableidentification of nonlinear (higher harmonics of the fundamentalfrequency) echo signals returned from tissue and microbubbles. Thesignal processor may also perform additional signal enhancement such asspeckle reduction, signal compounding, and noise elimination. Thebandpass filter in the signal processor can be a tracking filter, withits passband sliding from a higher frequency band to a lower frequencyband as echo signals are received from increasing depths, therebyrejecting the noise at higher frequencies from greater depths wherethese frequencies are devoid of anatomical information. The processedsignals are coupled to the B mode processor. The B mode processoremploys detection of an amplitude of the received ultrasound signal forthe imaging of structures in the body such as the tissue of organs andvessels in the body. B mode images of structures of the body may beformed in either the harmonic image mode or the fundamental image modeor a combination of both, as described in U.S. Pat. No. 6,283,919(Roundhill et al.) and U.S. Pat. No. 6,458,083 (Jago et al.). In variousembodiments, the detection unit 130 or the deriving unit 140 can beimplemented as part of the existing one or more processors, or as aseparate processor.

In order to derive the change in center frequency of the second harmoniccomponent along with the depth, the center frequency of the secondharmonic component at a depth along each scanning line is derived. Asknown by the skilled person, the center frequency of the second harmoniccomponent can be derived, for example, in the following way. The powerspectrum of the second harmonic component is derived using a slidingwindow having a first predetermined number of samples. The firstpredetermined number of samples can range from 50 to 500 samples.Preferably, the first predetermined number of samples is 150, i.e. thesliding window can have a size of 150 samples. In an example, thesliding window may move 2 samples at a time. The power spectrum of thesecond harmonic component at each depth can be computed based on thefirst predetermined number of samples, for example by using a 4096 or8192-point FFT (adding zeros after the first predetermined number ofsamples). In an example, the power spectrum can be averaged over anumber of RF lines, such as three RF lines, by means of “slidingaverage”. Thereafter, the center frequency of the second harmoniccomponent is computed from the power spectrum. For example, from thepower spectrum P_(l,d)(f) at the d-th depth along the l-th scanningline, the center frequency f_(l,d) ^(center) at d-th depth along thel-th scanning line is computed as:

$f_{l,d}^{center} = \frac{\int_{f}{{f \cdot {P_{l,d}(f)}}\ {f}}}{\int_{f}\; {{P_{l,d}(f)}\ {f}}}$

Since the attenuation/decrease of the center frequency due to a gaspocket at a specific depth is assumed to be greater than that due tosoft issue at the same depth, the change in the center frequency of thesecond harmonic component along with the depth can be used forindicating the existence of a gas pocket. For example, a gas pocketmight be detected in case of a sharply changed center frequency of thesecond harmonic component along with the depth, whereas soft tissuemight be detected in case of a nearly linearly changed center frequencyof the second harmonic component along with the depth.

In an aspect, a frequency-depth curve for the scanning line of theplurality of scanning lines in the ROI can be derived by determining thecenter frequency of the second harmonic component for each depth of thescanning line. Such frequency-depth curve for the scanning line canrepresent the relationship between the center frequency of the secondharmonic component and the depth and thus represent the change of thecenter frequency of the second harmonic component along with the depth.

Similarly, another frequency-depth curve for another scanning line ofthe plurality of scanning lines in the ROI can also be derived, and soon. As a result, for each scanning line of the plurality of scanninglines in the ROI, a respective frequency-depth curve can be derived. Assuch, the frequency-depth map for the ROI can be established based onall or a portion of the frequency-depth curves for the plurality ofscanning lines in the ROI.

According to an embodiment of the present invention, the frequency-depthcurve or map can be smoothened by averaging over a second predeterminednumber of scanning lines. For example, a smooth function such as amoving average function (also called sliding average function) can beapplied. Preferably, the second predetermined number ranges between 2and 50 lines. For example, the second predetermined number of scanninglines can be selected to be 50 lines.

Two typical frequency-depth curves are shown in FIG. 3a and FIG. 3b ,where the relationship between center frequency of the second harmoniccomponent and depth can be represented by such frequency-depth curve. Inthe examples of FIG. 3a and FIG. 3b , the second harmonic component isobtained by means of pulse inversion technique.

Referring back to FIG. 1, the deriving unit 140 is coupled to adetecting unit 150. The detecting unit 150 is configured to detect a gaspocket based on the change of the center frequency of the secondharmonic component in the depth direction. The detection unit 130, thederiving unit 140 and the detecting unit can be implemented as a singleprocessor or separate processors.

A display unit 160 is coupled to the deriving unit 140. The display unit160 is configured to generate an ultrasound image representing thederived change in the center frequency of the second harmonic componentalong with the depth.

Additionally or alternatively, the display unit 160 is coupled to thedetecting unit 150 and is configured to display, in an ultrasound image,an indicator for indicating the detected gas pocket. The ultrasoundimage can be of various types, such as a B-mode ultrasound image, anultrasound image representing the change of the center frequency alongwith the depth, or a combination thereof. The B-mode ultrasound imagemay represent the short-time energy of the echo signal or the short-timeenergy of the second harmonic component of the echo signal. Theshort-time energy may be derived using a sliding window. The size of thesliding window may range from 50 to 500 samples. For example, thesliding window can have a size of 150 samples and move 2 samples at atime.

The detecting unit 150 can be configured to detect the gas pocket invarious ways.

In an embodiment, in the case that the second harmonic component isobtained by means of pulse inversion technique, the detecting unit 150is configured to detect a gas pocket in the following way. If, in thedepth direction, the amount of change of the center frequency of thesecond harmonic component at a depth exceeds a first predeterminedthreshold, it is detected that there is a gas pocket at said depth.

Referring again to FIG. 3a and FIG. 3b , the relationship between centerfrequency of the second harmonic component and depth can be representedby each of the two frequency-depth curve. In an aspect, the change incenter frequency along with depth can be reflected by a slope of thefrequency-depth curve at a respective depth. We note that in FIG. 3a thecenter frequency drops sharply at a depth around 2.3 cm to 3.0 cm; whilein FIG. 3b the center frequency decreases nearly linearly along with thedepth.

Through analyzing data sets, the acoustical characteristics of a gaspocket and normal soft tissue can be determined through analysis ofultrasound echo signals. In one aspect, a sharp drop of the averagefrequency along with depth may be considered as being caused by a gaspocket. In another aspect, a nearly linearly average frequency alongwith depth may be considered as being caused by normal soft tissues. Forexample, in FIG. 3a and FIG. 3b , the frequency-depth curve in FIG. 3amay relate to a gas pocket, and the frequency-depth curve in FIG. 3b mayrelate to normal soft tissue (liver).

As described above, the gas pocket can be detected by comparing theamount of change in the center frequency of the second harmoniccomponent along with depth with a first predetermined threshold. If theamount of change at a depth exceeds the first predetermined threshold inan area, the area possibly contains a gas pocket; otherwise, the areapossibly contains soft tissues. The first predetermined threshold can beexperimentally derived. For example, the first predetermined thresholdcan be derived by performing a statistical analysis on the slopes of theaforementioned frequency-depth curve for the presence of a gas pocketand normal soft tissue. The absolution value of the slopes representsthe amount of change of the center frequency of the second harmoniccomponent.

A parametric slope map for the ROI can be generated to represent theslopes at the plurality of depths of the smoothened frequency-depthcurve for each scanning line. The slope values can be for example in MHzper cm. In an embodiment, the parametric slope map can be colorized toobtain a colorized parametric slope map, in which the slope value isrepresented by a color. A color bar is often provided adjacent to eachcolorized parametric map to indicate the correspondence between thecolors and the values indicated by the colors. Typically, the colorizedparametric map is colorized with various colors. As a particularexample, the parametric map can be colorized by various grey levels.

FIG. 4a shows a grey B-mode ultrasound image of a region of interest(ROI), FIG. 4b shows a colorized parametric ultrasound image of the ROI,and FIG. 4c shows the colorized parametric ultrasound image overlaidwith the grey B-mode ultrasound image. The colorized parametricultrasound image represents the colorized parametric slope map asdescribed above. Dynamic ranges for grey B-scan image and color map are60 dB and 0.3, respectively. In the example of FIGS. 4a-4c , the secondharmonic component is obtained by means of pulse inversion technique.

Referring to FIG. 4b , the blue color represents a smaller slope (i.e.the absolution value of the slope is higher) and thus a sharp drop ofthe center frequency of the second harmonic component, whilst the redcolor represents a higher slope. Since the sharp drop of the centerfrequency indicates that a gas pocket could exist, the blue areas (suchas areas 401) in FIG. 4b illustrate the locations of the possible gaspockets.

Referring to FIG. 4c , it shows the colorized parametric ultrasoundimage overlaid with the grey B-mode ultrasound image. Such an overlaidultrasound image allows the users to detect the gas pocket using boththe proposed approach based on the change of the center frequency of thesecond component and the conventional approach based on the B-modeultrasound image, resulting in a more robust detection.

In some cases, the absolute value of the slope at a tissue boundary canbe also very high, just like at a gas pocket. As is well-known, in theB-mode ultrasound image, the tissue boundary is normally darker or has alower amplitude, whilst the gas pocket is normally brighter or has ahigher amplitude. Through double checking the brightness and/or theamplitude in the B-mode ultrasound image, the gas pocket can bedistinguished from the tissue boundary. In other words, the gas pocketmay be brighter or has a higher amplitude than the tissue boundary.

In another embodiment, in the case that the second harmonic component isobtained by means of a band-pass filtering approach, the detecting unit150 is configured to obtain a first determining result indicatingwhether the change in the center frequency along with the depth in anarea forms a bell shape, and to determine whether a gas pocket ispresent in the area based on the first determining result. For example,if the center frequency of the second harmonic component in the middleof the area is higher than that in the other part of the area, it isdetermined that, in the area, the change of the center frequency alongwith the depth forms a bell shape. It will be appreciated by the skilledperson that the first determining result, namely whether the change inthe center frequency along with the depth in an area forms a bell shape,can be obtained in various ways such as by means of any suitable patternrecognition methods.

Additionally or alternatively, the detecting unit 150 is configured todetermine whether a gas pocket is present between a first depth and asecond depth along a scanning line based on a second determining result,wherein the second depth is deeper than the first depth, and the seconddetermining result indicates, along the scanning line, whether thechange in the center frequency along with the depth is greater than apositive second predetermined threshold at the first depth and less thana negative third predetermined threshold at the second depth. In anembodiment, the change in the center frequency along with the depth isrepresented by the slope of the frequency-depth curve. The skilledperson in the art will appreciate that more than one scanning line maygo across the same gas pocket, and the determined first and second depthalong each of multiple adjacent scanning lines can be used to determinethe outline of the gas pocket.

Experimental results show that the slope in the area of gas pocket isobviously larger than the slope in the area of surrounding normal softtissue. Therefore, the area of a gas pocket and the area of surroundingnormal soft tissue can be well differentiated based on the slope of thefrequency-depth curve, for example, by comparing the slope with thecorresponding predetermined thresholds.

In another embodiment, the detecting unit 150 is configured to obtain athird determining result indicating whether the intensity of theultrasound echo signal at the first depth is lower than a fourththreshold, and to determine whether a gas pocket is present between thefirst depth and the second depth based on the second determining resultand the third determining result. Preferably, the fourth threshold isdetermined based on the average intensity over the whole ROI. Theintensity of the ultrasound echo signal can be, for example, representedby the short-time energy of the ultrasound echo signal, such as theimage value of a regular B-mode image.

In particular, if the second determining result indicates there might bea gas pocket, but the third determining result indicates that theintensity of the ultrasound echo signal at the first depth is lower thanthe fourth threshold, the detecting unit 150 is configured so as not todetermine there is a gas pocket.

In this way, the detecting accuracy can be improved for the followingconsiderations. If the averaged intensity in an area after a positiveslope is too low as compared to the averaged intensity in the wholeimage, said area is expected to be a near-field area associated withskin and fat, or a boundary, or there is too much noise or too manyartifacts.

FIGS. 5a-5e shows an example in which the ultrasound echo signal iscollected along 256 scanning lines in a region of interest (ROI), andthe second harmonic component of the ultrasound echo signal is obtainedby means of band-pass filtering. FIG. 5a shows a grey B-mode ultrasoundimage of the ROI, FIG. 5b shows one colorized parametric ultrasoundimage of the ROI representing the center frequency of the secondharmonic component, FIG. 5c shows another colorized parametricultrasound image of the ROI representing the slope of the centerfrequency of the second harmonic component, FIG. 5d shows thefrequency-depth curve along the 150^(th) scanning line in the ultrasoundimage of FIG. 5b , and FIG. 5e shows the frequency-depth curve at adepth of 4 cm in the ultrasound image of FIG. 5 b.

Referring to FIG. 5b , the x-axis represents the indices of the scanningline, the y-axis represents the depth in cm, and the color representsthe center frequency of the second harmonic component of the ultrasoundecho signal received from the ROI in MHz, as indicated by the color barat one side. It can be seen from FIG. 5b that the center frequencygenerally decreases along with the depth, but there is an increasefollowed by a further decrease in area 501 and thereby a bell shape isformed in area 501. As described in the above, based on such a bellshape, it can be determined either manually (e.g. by a clinician'sobservation) or automatically (e.g. by the detecting unit 150) that agas pocket is present in area 501. FIG. 5d shows the frequency-depthcurve along the 150th scanning line in the ultrasound image of FIG. 5b .It can be clearly observed in FIG. 5d that the frequency-depth curveforms a bell shape approximately between 3.5 cm and 5 cm.

Referring to FIG. 5c , the x-axis represents the indices of the scanningline, the y-axis represents the depth in cm, and the color representsthe slope of the center frequency of the second harmonic component alongeach scanning line, as indicated by the color bar at one side. It can beseen from FIG. 5c that the slope is generally in the range of [−0.2,0.2] in the whole ROI, but the slope is approximately 0.5 at around 3.5cm (as indicated by reference sign 502) and is approximately −0.5 atabout 5 cm (as indicated by reference sign 503). As described in theabove, based on such information, it can be determined either manually(e.g. by the clinician's observation) or automatically (e.g. by thedetecting unit 150) that a gas pocket is present approximately between3.5 cm and 5 cm.

Referring now to FIG. 6, a method 600 of detecting a gas pocket in theROI is illustrated. While, for the sake of simplicity of explanation,the method is shown and described as a series of steps, it is to beunderstood and appreciated that the methodology is not limited by theorder of steps, as some steps may occur, in accordance with one or moreaspects, in different orders and/or concurrently with other steps ascompared to the orders and steps shown and described herein. Forexample, those skilled in the art will understand and appreciate that amethodology could alternatively be represented as a series ofinterrelated states or events, such as in a state diagram. Moreover, notall illustrated steps may be utilized to implement a method inaccordance with the claimed subject matter. In general, a process can beimplemented as processor instructions, logical programming functions, orother electronic sequence that supports the detecting of gas pocketsdescribed herein.

FIG. 6 is an example gas pocket detection method 600 for an ultrasoundsystem. The method 600 comprises step 10, step 20, step 30 and step 40.

At step 10, an ultrasound probe 110 is operated to transmit anultrasound signal toward the ROI and to acquire an ultrasound echosignal reflected from the ROI along a plurality of scanning lines.

At step 20, a second harmonic component of the ultrasound echo signalmay be obtained for each depth of a plurality of depths along each ofthe plurality of scanning lines. In an aspect, the second harmoniccomponent is obtained by means of pulse inversion technique.

At step 30, a change of the center frequency of the second harmoniccomponent along with the depth is derived, wherein the change in thecenter frequency of the second harmonic component along with the depthcan be used to indicate the existence of a gas pocket. For example, agas pocket might be detected in case of a sharply changed centerfrequency of the second harmonic component along with the depth, whereassoft tissue might be detected in case of a nearly linearly changedcenter frequency of the second harmonic component along with the depth.

In an aspect, step 30 comprises deriving, for each scanning line, afrequency-depth curve representing the relationship between the centerfrequency of the second harmonic component and the depth and derivingthe slope of the frequency-depth curve at each depth. According to oneexample, the frequency-depth curve is smoothened by averaging over asecond predetermined number of ultrasound scanning lines.

In another aspect, method 600 further comprises a step of detecting agas pocket based on the change in the center frequency along with thedepth. In an example, if an amount of the change in the center frequencyalong with the depth exceeds, at a depth, a first predeterminedthreshold, the gas pocket is detected at said depth. In another example,if the change in the center frequency along with the depth forms a bellshape in an area, it is detected that a gas pocket is present in thearea.

Additionally, whether the detected gas pocket is normal or abnormal canbe further determined based on the detected gas pocket and furtherinformation.

In an aspect, the further information may relate to the location of thegas pocket and to what type of tissue it is surrounded by. For example,a gas pocket surrounded by a liver-like texture might be abnormal sincesuch a gas pocket is not expected to be seen there. According to anotherexample, if the surrounding tissue of a gas pocket has a bowel-likeappearance, then the gas pocket might be normal. According to yetanother example, if the surrounding tissue of a gas pocket is not

bowel-like, then, in general, the gas pocket might be abnormal.

At step 40, an ultrasound image is displayed.

In an embodiment, at step 40, an ultrasound image is generated based onthe derived change in the center frequency of the second harmoniccomponent along with the depth and then displayed. For example, theultrasound image can be a colorized parametric ultrasound image as shownin FIG. 4b or FIG. 5c . In another embodiment, at step 40, an ultrasoundimage is generated, based on the center frequency of the second harmoniccomponent along with the depth, and then displayed. For example, theultrasound image can be a colorized parametric ultrasound image as shownin FIG. 5 b.

Additionally or alternatively, at step 40, an indicator for indicatingthe detected gas pocket is displayed in an ultrasound image which may beof various types, such as a B-mode ultrasound image, an ultrasound imageillustrating the change of the center frequency along with the depth, ora combination thereof. For example, the ultrasound image can be a greyB-mode ultrasound image as shown in FIG. 4a or FIG. 5a , a colorizedparametric ultrasound image as shown in FIG. 4b or FIG. 5b or FIG. 5c ,or the colorized parametric ultrasound image overlaid with the greyB-mode ultrasound image as shown in FIG. 4 c.

The technique processes described herein may be implemented by variousmeans. For example, these techniques may be implemented in hardware,software, or a combination thereof. For a hardware implementation, theprocessing units may be implemented within one or more applicationspecific integrated circuits (ASICs), digital signal processors (DSPs),digital signal processing devices (DSPDs), programmable logic devices(PLDs), field programmable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described herein, or a combination thereof. Withsoftware, implementation can be through modules (e.g., procedures,functions, and so on) that perform the functions described herein. Thesoftware codes may be stored in a memory unit and executed by theprocessors.

Moreover, aspects of the claimed subject matter may be implemented as amethod, apparatus, system, or article of manufacture using standardprogramming and/or engineering techniques to produce software, firmware,hardware, or any combination thereof to control a computer or computingcomponents to implement various aspects of the claimed subject matter.The term “article of manufacture” as used herein is intended toencompass a computer program accessible from any computer-readabledevice, carrier, or media. For example, computer readable media caninclude but are not limited to magnetic storage devices (e.g., harddisk, floppy disk, magnetic strips . . . ), optical disks (e.g., compactdisk (CD), digital versatile disk (DVD) . . . ), smart cards, and flashmemory devices (e.g., card, stick, key drive . . . ). Of course, thoseskilled in the art will recognize that many modifications may be made tothis configuration without departing from the scope or spirit of what isdescribed herein.

As used in this application, the term “unit” such as “obtaining unit”,“deriving unit”, “detecting unit” is intended to refer to a processor ora computer-related entity, either hardware, a combination of hardwareand software, software, or software in execution. For example, acomponent may be, but is not limited to, a process running on aprocessor, a processor, an object, an executable, a thread of execution,a program, and/or a computer. By way of illustration, both anapplication running on a server and the server can be a component. Oneor more components may reside within a process and/or thread ofexecution and a component may be localized on one computer and/ordistributed among two or more computers.

What has been described above includes examples of one or moreembodiments. It is, of course, not possible to describe everyconceivable combination of components or methodologies for the purposeof describing the aforementioned embodiments, but one of ordinary skillin the art may recognize that many further combinations and permutationsof various embodiments are possible. Accordingly, the describedembodiments are intended to embrace all such alterations, modificationsand variations that fall within the spirit and scope of the appendedclaims. Furthermore, to the extent that the term “includes” is used ineither the detailed description or the claims, such term is intended tobe inclusive in a manner similar to the term “comprising” as“comprising” is interpreted when employed as a transitional word in aclaim.

1. An ultrasound system for detecting a gas pocket in aRegion-of-Interest, comprising: an ultrasound probe for transmitting anultrasound signal toward the Region-of-Interest and acquiring anultrasound echo signal reflected from the Region-of-Interest along aplurality of scanning lines; an obtaining unit for obtaining a secondharmonic component of the ultrasound echo signal for each depth of aplurality of depths along each scanning line of the plurality ofscanning lines; and a deriving unit for deriving a change in a centerfrequency of the second harmonic component along with the depth.
 2. Theultrasound system according to claim 1, wherein the second harmoniccomponent is obtained by means of pulse inversion technique.
 3. Theultrasound system according to claim 1, wherein the second harmoniccomponent is obtained by means of band-pass filtering.
 4. The ultrasoundsystem according to claim 1, wherein the deriving unit is configured toderive, for each scanning line, a frequency-depth curve representing therelationship between the center frequency and the depth; and to derive aslope of the frequency-depth curve at each depth of the plurality ofdepths.
 5. The ultrasound system according to claim 4, wherein thefrequency-depth curve is smoothened by averaging over a secondpredetermined number of ultrasound scanning lines.
 6. The ultrasoundsystem according to claim 1, further comprising: a display unit forgenerating an ultrasound image representing the derived change in thecenter frequency along with the depth and displaying the ultrasoundimage.
 7. The ultrasound system according to claim 1, furthercomprising: a detecting unit for detecting a gas pocket based on thechange in the center frequency along with the depth; and a display unitfor displaying, in an ultrasound image, an indicator for indicating thedetected gas pocket.
 8. The ultrasound system according to claim 7,wherein the second harmonic component is obtained by means of pulseinversion technique; and the gas pocket is detected at a depth if anamount of the change in the center frequency along with the depthexceeds a first predetermined threshold at the depth.
 9. The ultrasoundsystem according to claim 7, wherein the second harmonic component isobtained by means of band-pass filtering; and the detecting unit isconfigured to obtain a first determining result indicating whether thechange in the center frequency along with the depth in an area forms abell shape, and to determine whether a gas pocket is present in the areabased on the first determining result.
 10. The ultrasound systemaccording to claim 7, wherein the second harmonic component is obtainedby means of band-pass filtering; and the detecting unit is configured todetermine whether a gas pocket is present between a first depth and asecond depth along a scanning line based on a second determining result,wherein the second depth is deeper than the first depth, and the seconddetermining result indicates, along the scanning line, whether thechange of the center frequency along with the depth is greater than apositive second predetermined threshold at the first depth and less thana negative third predetermined threshold at the second depth.
 11. Theultrasound system according to claim 10, wherein the detecting unit isconfigured to obtain a third determining result indicating whether theintensity of the ultrasound echo signal at the first depth is lower thana fourth threshold, and to determine whether a gas pocket is presentbetween the first depth and the second depth along the scanning linebased on the second determining result and the third determining result.12. The ultrasound system according to claim 7, further comprising: adetermining unit for determining if the detected gas pocket is normal orabnormal based on the detected gas pocket and further information,wherein the further information relates to location of the gas pocketand information on what type of tissue surrounds the gas pocket.
 13. Amethod of detecting a gas pocket in a Region-of-Interest, comprisingsteps of: transmitting (step 10) an ultrasound signal toward theRegion-of-Interest and acquiring (step 10) an ultrasound echo signalreflected from the Region-of-Interest along a plurality of scanninglines; obtaining (step 20) a second harmonic component of the ultrasoundecho signal for each depth of a plurality of depths along each scanningline of the plurality of scanning lines; and deriving (step 30) a changein a center frequency of the second harmonic component along with thedepth.
 14. The method according to claim 13, further comprising stepsof: generating (step 40) an ultrasound image based on the derived changein the center frequency along with the depth; and displaying (step 40)the ultrasound image.
 15. The method according to claim 13, furthercomprising steps of: detecting a gas pocket based on the change in thecenter frequency along with the depth; and displaying, in an ultrasoundimage, an indicator for indicating the detected gas pocket.